Excellent and original
Rarely do I come across a game that I think is this fun and flawless. It's quite strategic, not too frustrating and simple to grasp. A great way to waste an hour on a Sunday, most certainly.
As for improvements, the Sniper Bot was a little broad - I clicked a light and it shot the man below it, even though there was a large gap between the two of them. Limiting the area in which you can click a person to kill them would reduce this as well as making it more of a challenge.
I found the new guards at the end of the game a little irritating, as they were all too excellent - they could just destroy everything I threw at them unless I overwhelmed or shot them, and they took out a lot of the strategy that existed before.
Perhaps to solve a lot of the issues, there could be a certain amount of time needed for someone to unlock a door or fix a light? Or perhaps mechanics wouldn't try to unlock a door unless it became an issue for him or someone else in his line of sight to open it? At the moment they're basically wizards.
Other than that, all I can suggest is just expanding the game - bigger maps, more to do, maybe even doors that go horizontally, or lifts between floors, so as the game goes on you need to be more tactical to trap people in certain areas? Maybe even a more story-based mode with certain objectives that runs parallel to the sandbox mode?
One of the best games to come to NG lately, I'll be shocked if it isn't on the front page.